This invention is generally related to lock and key sets having a rotating cylinder lock and is particularly directed to an electronic interlock to be used in conjunction with a rotating cylinder lock mechanism.
Over the last several years, it has become increasingly desirable to improve the anti-tampering features of lock and key sets. This is particularly true with respect to automobile ignition systems where auto theft has almost developed into an art form. Skilled thieves can often "hot wire" an automobile ignition in a matter of a few seconds. Typically, the key and cylinder lock for engaging and energizing the ignition system is either bypassed or pulled in order to facilitate the theft. To combat this, automotive manufacturers have incorporated a variety of vehicular anti-tampering systems (VATS) to make vehicle theft more difficult. Numerous of these include electrical or electronic interlocks working in cooperation with a mechanical lock system. For example, one such system includes a resistor element on the mechanical key and a circuit connection contained within the cylinder of the key lock. When a mated key with the proper resistance level is inserted in the cylinder, the circuit is closed and the proper coded voltage is produced, permitting the ignition to energize in typical fashion when the cylinder is rotated. If a key with an improper resistance level is used, the proper voltage is not produced and rotation of the cylinder will not enable the ignition system.
In another example, a sensor is placed at a certain point in the rotation of the cylinder and senses proper rotation of the cylinder to produce an ignition activation signal. Any attempt to start the ignition without first properly rotating the cylinder is ineffective since proper rotation is required to generate the ignition activation signal. Efforts have been made to override the electronic interlock by deciphering the coded resistance values and duplicating them in order to engage the ignition.
With the development and availability of onboard computer systems, electronic interlocks are becoming more widely available and more sophisticated at a rapid rate. For example, if an attempt is made to duplicate a resistance level required to deactivate an electronic interlock, and the attempt is not successful, the computer system can be programmed to shut down the ignition circuitry for a delay period of 2.5 minutes more or less. If ten resistance levels are available for a particular car system, the thief must try as many as ten different duplicates before he can be assured of starting the car. On the average, this would increase the amount of time it takes to "hot wire" a car from a few seconds to ten to fifteen minutes or more. In many cases, a ten minute delay is more than sufficient to foil a theft attempt.
While the need for VATS ignitions in automobiles has created the development of the electronic interlock technology, it will be readily apparent that there is a wide variety of uses for which the interlock systems can be incorporated. The electronic interlock systems for vehicle ignition circuits are readily adaptable to any lock and key set utilizing a key with a rotating cylinder lock.
While the systems of the prior art have greatly enhanced the anti-theft features of lock systems, it is desirable to improve upon the systems by making it more difficult, if not impossible, for a thief to read and decode the electronic or resistance level codes utilized in connection with the interlock. In this regard, developing technology includes a fail safe system in combination with the interlock for precluding unauthorized decoding of the interlock code by blocking the signal whenever an attempt is made to unlock the lock in an unauthorized manner. One example of such a system is illustrated as prior art in FIG. 1 of the drawing. As there shown, an electronic interlock system comprising a sensor circuit in series with a coded resistor and a fail safe system is coupled to a VATS module provided by the automobile manufacturer. When a mated key is inserted in the ignition lock cylinder and the cylinder is rotated, a specific point on the cylinder passes by a sensor generating a readable signal which is introduced into the comparator circuit of the VATS module. When the signal is first received by the comparator, it is programmed into the memory and thereafter, the generated signal is compared with the stored signal to determine the presence of an acceptable ignition sequence. Upon an acceptable comparison, the ignition circuitry and fuel delivery system are energized and the vehicle may be started. The fail safe system of FIG. 1 includes a diode in series with the coded resistor for precluding unauthorized reading of the resistor level when a reverse voltage is placed across terminals B and C. While this system is successful in precluding the unauthorized reading of the coded resistor, it has several disadvantages. First, by using a fail safe system that is in series with the coded resistor, the voltage drop across the diode becomes part of the decoded signal read by the reader. In addition, most semi-conductor diodes are temperature sensitive, the coded signal varies substantially depending on ambient conditions. This requires that the width for each coded signal be increased, reducing the number of codes available to the interlock.